In covalent bonding electrons are shared by the two elements. There are single, double and triple bonds.

Bonds can be polar or non-polar. Polar means opposite. Opposite in chemistry means positive or negative. Every element has a different electronegativity which is the strength of that atom at pulling electrons towards itself. So if one element is stronger at pulling electrons, it has more of the shared pair of electrons, and since electrons are negative, it will be partially negative; whereas, the weaker element will have less of the shared pair of electrons, so it will be partially positive.